Nonaqueous secondary battery with lithium titanium cathode

ABSTRACT

A lithium secondary cell comprising a positive electrode containing a lithium titanate as an active material, a negative electrode containing a carbonaceous material as an active material, and an electrolytic solution comprising a solution of a lithium salt in an organic solvent. The lithium titanate preferably has a composition of the formula:(0.8&lt;=x&lt;=1.4 and 1.6&lt;=y&lt;=2.2). The lithium secondary cell has a high capacity suitable for use as a power source for a wristwatch and good charge-discharge properties, at a nominal voltage of 1.5 V.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/JP97/02008 which has an Internationalfiling date of Jun. 11, 1997 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

FILED OF THE INVENTION

The present invention relates to a lithium secondary cell. Inparticular, the present invention pertains to lithium secondary cellssuitable as backup batteries for watches, power sources for portabledevices such as pagers, timers, etc., backup batteries for memories, andthe like. PRIOR ART

Lithium secondary cells comprising a negative electrode made of metallithium or a lithium alloy have been mainly developed as secondarycells. However, when the metal lithium or lithium alloy is contained inthe negative electrode, lithium ions in an electrolyte tend toprecipitate in the form of metal lithium on the negative electrodeduring recharging. The deposited lithium forms minute particles or growslithium dendrites on the surface of the negative electrode, and causes ashort-circuit in the cell. Thus, the charge-discharge cycle life of thecell is shortened. Consequently, lithium cells, which use neither metallithium nor a lithium alloy in the negative electrode and have a highenergy density, have been studied.

Currently, primary cells such as silver oxide cells are used as powersources for wristwatches. However, the primary cells suffer fromproblems associated with disposal of the used cells. Thus, wristwatcheshaving built-in power generators, which require no replacement of thecells, have been developed, and electric double layer capacitors areused as power sources used in such wristwatches. However, the electricdouble layer capacitors have a small capacity per unit volume, and thusit is desired to develop substitute power sources for the electricdouble layer capacitor.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a lithium secondarycell which can be charged and discharged, does not suffer from theproblem of disposal, and has a high capacity so that it is suitable as apower source for a wristwatch.

Accordingly, the present invention provides a lithium secondary cellcomprising a positive electrode containing lithium titanate as an activematerial, a negative electrode containing a carbonaceous material as anactive material, and an electrolytic solution comprising a solution of alithium salt in an organic solvent.

Since a lithium titanate as a positive electrode active material and acarbonaceous material as a negative electrode active material are usedin combination, the lithium ions can be easily doped and dedoped at anominal voltage of 1.5 V, and thus a lithium secondary cell having ahigh capacity and good charge-discharge cycle properties is obtained.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a cross section of an example of a lithium secondary cellaccording to the present invention.

FIG. 2 is a graph showing the discharge properties of the cells ofExamples 1 and 2 and comparative Example 1 in the first discharge.

FIG. 3 is a graph showing the charge-discharge cycle properties of thecells of Examples 1 and 2 and comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

A lithium titanate used as a positive electrode active materialaccording to the present invention can be prepared by heating titaniumoxide and a lithium compound at a temperature of between 760 and 1100°C.

In general, the lithium titanate is represented by the formula (1):

Li_(x)Ti_(y)O₄  (1)

Usually, x and y in the above formula are numerals in the range between0.8 and 1.4 (0.8≦x≦1.4), and between 1.6 and 2.2 (1.6≦y≦2.2),respectively. In particular, the lithium titanate of the formula (1) inwhich x is 1.33 and y is 1.67 is preferable.

Titanium oxide may be either anatase or rutile. The lithium compound maybe lithium hydroxide, lithium carbonate, lithium oxide, and the like.

A positive electrode is preferably prepared by mixing lithium titanate,a conducting aid and a binder to obtain a positive electrode mixture,and shaping the mixture under pressure.

Examples of the conducting aid are scaly graphite, acetylene black,carbon black, and the like. Fluororesins are preferably used as binders.Examples of fluororesins are polytetrafluoroethylene, polyvinylidenefluoride, and the like.

Proportions of the components constituting the positive electrode arepreferably 70 to 90 wt. % of the lithium titanate as the positiveelectrode active material, 5 to 20 wt. % of the conducting aid, and 1 to10 wt. % of the binder.

When the amount of the lithium titanate is less than the above lowerlimit, the capacity of the cell tends to decrease and high capacity maynot be achieved. When the amount of the lithium titanate exceeds theabove upper limit, the amounts of the electrical conducting aid andbinder decrease correspondingly, and the conductivity or strength of thepositive electrode mixture may decrease.

When the amount of the conducting aid is less than the above lowerlimit, the electrical conductivity may decrease. When the amount of theconducting aid exceeds the above upper limit, the amount of the lithiumtitanate decreases correspondingly and the capacity of the cell maydecrease.

When the amount of the binder is less than the above lower limit, theintegrity of the positive electrode mixture may decrease and shaping ofthe mixture may become difficult. When the amount of the binder exceedsthe above upper limit, the amount of the lithium titanate decreasescorrespondingly and the capacity of the cell may decrease.

The production method for the positive electrode is not limited to theabove method, and compositions of the components are not limited to theabove described one.

A negative electrode is preferably prepared by mixing a carbonaceousmaterial as a negative electrode active material and a binder to obtaina negative electrode mixture, and shaping the mixture under pressure.

Examples of the carbonaceous material as the negative electrode activematerial are synthetic graphite, natural graphite, low crystallinecarbon, coke, anthracite (hard coal), and the like. In particular,synthetic graphite is preferable, since it can achieve the highercapacity than other carbonaceous materials.

Fluororesins are preferably used as binders. Examples of fluororesinsare polytetrafluoroethylene, polyvinylidene fluoride, and the like.

The proportions of the components constituting the negative electrodeare preferably 80 to 95 wt. % of the carbonaceous material as thenegative electrode active material and 5 to 20 wt. % of the binder.

When the amount of the carbonaceous material as the negative electrodeactive material is less than the above lower limit, it may be difficultto obtain a lithium secondary cell having a high capacity. When theamount of the carbonaceous material exceeds the above upper limit, theamount of the binder decreases, and thus the integrity of the positiveelectrode mixture may decrease and shaping of the mixture may becomedifficult.

The production method for the negative electrode is not limited to theabove method, and compositions of the components are not limited to theabove described one. For example, a conducting aid may be added to thenegative electrode mixture.

In the present invention, the cell comprises an electrolytic solutionwhich is prepared by dissolving a lithium salt in an organic solvent.Examples of the organic solvent used as the solvent for the electrolyticsolution are propylene carbonate, ethylene carbonate, butylenecarbonate, γ-butyrolactone, 1,2-dimethoxyethane, dimethoxymethane,tetrahydrofuran, dioxolane, and the like.

Examples of the lithium salt are LiN(CF₃SO₂)₂, LiClO₄, LiPF₆, LiBF₄,LiAsF₆, LiSbF₆, LiCF₃SO₃, LiCF₃CO₂, LiC_(n)F_(2n+1)SO₃ (n≧2),LiN(CF₃CF₂SO₂)₂, and the like. Among them, LiN(CF₃SO₂)₂, LiPF₆, LiCF₃SO₃and LiBF₄ are preferably used since they have high conductivity, and arethermally stable.

The concentration of the lithium salt in the electrolytic solution isnot limited, and is usually between 0.1 and 2 mole/l, preferably between0.4 and 1.4 mole/l.

The structure and production method of the lithium secondary cell of thepresent invention are substantially the same as those of conventionallithium secondary cells except that the above positive and negativeelectrodes and electrolytic solution are used.

EXAMPLES

The present invention will be illustrated by the following Examples,which do not limit the scope of the present invention in any way.

Example 1

Anatase titanium oxide (2 moles) and lithium hydroxide (1 mole) weremixed and calcined using an electric furnace in air at 800° C. for 8hours, and a lithium titanate was obtained. The composition of thislithium titanate was analyzed by atomic absorption analysis, and foundto be Li_(1.33)Ti_(1.67)O₄.

The obtained lithium titanate (100 wt. parts), carbon black (5 wt.parts) and graphite (5 wt. parts) as conducting aids, andpolytetrafluoroethylene (5 wt. parts) as a binder were mixed inisopropanol to prepare a positive electrode mixture. After evaporatingoff the solvent, the positive electrode mixture was molded in the formof a pellet having a diameter of 6.0 mm and a thickness of 0.5 mm. Thepellet was dried and dehydrated with a far-infrared drier at 250° C. for30 minutes to form a positive electrode.

Separately, synthetic graphite (90 wt. parts) and polyvinylidenefluoride (10 wt. parts) as a binder were mixed in N-methylpyrrolidone toprepare a negative electrode mixture. After evaporating off the solvent,the negative electrode mixture was molded in the form of a pellet havinga diameter of 3.5 mm and a thickness of 1.0 mm. The pellet was dried anddehydrated with a far-infrared drier at 120° C. for 30 minutes to form anegative electrode.

An electrolytic solution, which was prepared by dissolving LiN(CF₃SO₂)₂in a mixed solvent of ethylene carbonate and diethyl carbonate in avolume ratio of 1:1 at a concentration of 1.0 mole/l, was used.

Using the above positive and negative electrodes and electrolyticsolution, a lithium secondary cell having the structure shown in FIG. 1and an outer diameter of 6.7 mm and a height of 2.1 mm was assembled.

In FIG. 1, a positive electrode 1 consisted of a press molded article ofa positive electrode mixture containing the lithium titanate(Li_(1.33)Ti_(1.67)O₄) as an active material, carbon black and graphiteas conducting aids, and polytetrafluoroethylene as a binder.

A negative electrode 2 consisted of a press molded article of a negativeelectrode mixture containing synthetic graphite as an active materialand polyvinylidene fluoride as a binder.

A separator 3 made of polypropylene non-woven fabric was insertedbetween the positive electrode 1 and negative electrode 2.

During assembling of the cell, the negative electrode 2 was doped withlithium ions in the presence of an electrolytic solution while placingmetal lithium in an amount corresponding to 80% of the electric capacityof the positive electrode on the opposite side of the separator 3.

The positive electrode 1, negative electrode 2, separator 3 andelectrolytic solution were sealed in a space formed by a positiveelectrode can 4 made of stainless steel, a negative electrode can 5 madeof stainless steel, and an insulation packing 6 made of polypropylene.

Example 2

A lithium secondary cell was produced in the same manner as in Example 1except that an electrolytic solution, which had been prepared bydissolving LiPF₆ in place of LiN(CF₃SO₂)₂ in a mixed solvent of ethylenecarbonate and diethyl carbonate in a volume ratio of 1:1 at aconcentration of 1.0 mole/l, was used.

Comparative Example 1

A lithium secondary cell was produced in the same manner as in Example 1except that lithium iron oxide (LiFe₅O₈) was used as a positiveelectrode active material in place of a lithium titanate.

Each of the cells produced in Examples 1 and 2 and Comparative Example 1was charged and discharged under the following conditions, and thedischarge property in the first discharge, and the charge-dischargecycle property were evaluated:

Charging conditions: constant current, 0.1 mA and charge cut current,2.4 mA

Discharging conditions: constant current, 0.1 mA and discharge cutcurrent, 0.4 mA

The discharge properties in the first discharge are shown in FIG. 2, andthe charge-discharge properties are shown in FIG. 3.

As seen from FIG. 2, the cells of Examples 1 and 2 had better flatnessof the cell voltage around 1.5 V and larger cell capacity down to 0.4 Vthan the cell of Comparative Example 1. Thus, the cells of Examples 1and 2 had a high capacity.

As seen from FIG. 3, the cells of Examples 1 and 2 had the larger cellcapacity than the cell of Comparative Example 1 after the same number ofcycles. Furthermore, the former cells suffered less decrease of cellcapacity due to the increase of the number of cycles than the lattercell. That is, the cells of Examples 1 and 2 had good charge-dischargecycle properties.

In contrast, the cell of Comparative Example 1 had a small cellcapacity, and the cell capacity dropped sharply in the early cycles ofcharge and discharge. That is, this cell had low charge-discharge cycleproperties. These properties of the cell of Comparative Example 1 may beattributed to the destabilization of the crystal structure of lithiumiron oxide, which was used as the positive electrode active material,during the charge and charge cycles.

A lithium secondary cell was produced in the same manner as in Example 1or 2 using the same electrolytic solution and negative electrode, andthe lithium titanate having the composition of Li₁Ti₂O₄ orLi_(0.8)Ti_(2.2)O₄ in place of Li_(1.33)Ti_(1.67)O₄, and a cell capacityand charge-discharge cycle properties were evaluated. The results werethe same as those in Examples 1 and 2.

As explained above, the present invention can provide lithium secondarycells having a high capacity and good charge-discharge cycle propertiesat a nominal voltage of 1.5 V, since a lithium titanate of the formula:Li_(x)Ti_(y)O₄ is used as a positive electrode active material, and acarbonaceous material such as synthetic graphite is used as a negativeelectrode active material.

What is claimed is:
 1. A lithium secondary cell comprising a positiveelectrode containing lithium titanate as an active material, a negativeelectrode containing a carbonaceous material as an active material, andan electrolytic solution comprising a solution of a lithium salt in anorganic solvent, wherein said positive electrode, said negativeelectrode and said electrolytic solution are sealed in a space formed bya positive electrode can, a negative electrode can and an insulationpacking, said lithium titanate has a composition represented by theformula: Li_(x)Ti_(y)O₄ wherein x and y are in the range between 0.8 and1.4 (0.8≦x≦1.4), and between 1.6 and 2.2 (1.6≦y≦2.2), respectively, andlithium ions are doped and dedoped at a nominal voltage of 1.5 V.
 2. Thelithium secondary cell according to claim 1, wherein x is 1.33 and y is1.67.
 3. The lithium secondary cell according to claim 1, wherein saidpositive electrode comprises said lithium titanate, a conducting aid anda binder.
 4. The lithium secondary cell according to claim 3, whereinproportions of components constituting said positive electrode are 70 to90 wt. %.
 5. The lithium secondary cell according to claim 1, whereinsaid negative electrode comprises graphite and a binder.
 6. The lithiumsecondary cell according to claim 1, wherein said lithium salt is atleast one lithium salt selected from the group consisting ofLiN(CF₃SO₂)₂, LiPF₆, LiCF₃SO₃ and LiBF₄.
 7. The lithium secondary cellaccording to claim 1, wherein the lithium titanate is formed by aprocess comprising the following steps: mixing titanium oxide andlithium hydroxide in a molar proportion of about 2 to 1 of the titaniumoxide to the lithium hydroxide; and calcining at about 800° C. for about8 hours.
 8. The lithium secondary cell according to claim 7, wherein thecalcining is performed in air.
 9. The lithium secondary cell accordingto claim 7, wherein the calcining is performed using an electricfurnace.
 10. The lithium secondary cell according to claim 1, whereinthe positive electrode is formed by a process comprising the followingsteps: mixing 100 parts (by weight) Li_(1.33)Ti_(1.67)O₄, 5 parts (byweight) carbon black, 5 parts (by weight) graphite and 5 parts (byweight) polytetrafluoroethylene in an isopropanol solvent; evaporatingoff the solvent to form a mixture; molding the mixture into a pellet;and drying the pellet at about 250° C. for about 30 minutes.